1. Field of the Invention
The present invention relates to a method and system for biofouling control in, for example, the intake structure of a power plant.
2. Discussion of Background
A major problem in power plant cooling is biofouling--the deposition of organic material on cooling system surfaces. The detrimental effects of biofouling are reduced heat transfer capability in condensers, increased pressure drop or lower flow, and accelerated corrosion. The mechanisms of biofouling are complex, involving biological, physical, and chemical interactions. Its proliferation is dependent on several factors: water quality, water-borne substances and organisms, plant process conditions, interactions with other cooling system fouling phenomena, and the control measures employed.
Because of low to moderate flow velocities, isolated pockets of quiescent zones, and warm temperatures in the condensers, a plant cooling system provides an ideal environment for biological colonization and growth. Two forms of biofouling are significant: microbiological fouling of heat exchangers and macroinvertebrate fouling of the intake structure. Organisms responsible for microbiological fouling include: slime and alga. Macroinvertebrates include: mussels, barnacles, oysters and clams.
It has been determined that microbiological fouling is a dominant problem, representing approximately 70 percent of those power plant units which experienced biofouling in one form or another. In recent years, episodes of macroinvertebrate or macrofouling have risen and have become the second most important cooling system biofouling control issue in the electric power industry.
The penalties and costs associated with impaired condenser operations are substantial. They include increased fuel consumption, replacement power costs, condenser cleaning costs, and loss of availability. It is estimated that the effect of biofouling on unit availability for fossil-fired power plants over 600 MW is a 3.8 percent loss in unit availability directly attributable to poor reliability of condenser systems. The loss in availability of a typical baseload 600 MW coal unit cost an average of $500,000 a day in 1982. Availability losses from condenser problems are usually in the nature of partial load reductions. The modular nature (divided water boxes) of most condensers permits inspection, leak detection, tube plugging, and condenser cleaning while the unit is on-line.
Performance losses resulting from tube blockage and cooling water flow reductions are also significant. The direct effects of these macrofouling-based problems can only be determined on a unit by unit basis. The effects may also vary with time as a result of fouling seasons. If these effects can be correlated with an annual yearly increase in back pressure, the economic impact for typical units can be determined. It is not unreasonable to assume an average yearly increase of 0.3-inch Hg which correlates to annual replacement energy costs of approximately $800,000 for a 1150 MWe nuclear unit, and approximately $95,000 for a 600 MWe coal fired fossil unit.
Since the 1920's, gaseous chlorine or liquid hypochlorite solutions have been used to control both microbiological and macrobiological fouling at power plants. Chlorine is a powerful oxidizing agent and an effective bactericide. It eliminates bacterial slime through its toxicity to the microorganisms and through its chemical reaction with the organic substances forming the slime matrix. Chlorine is effective in controlling macrobiological fouling by having direct toxic effects on adult organisms, by inhibiting the settlement of larval stages, and by weakening the mechanisms by which the organisms remain attached to the cooling system substrates.
For the effective microbiofouling control chlorine is applied to power plant cooling water on an intermittent basis. Dosing frequency, duration, and concentration vary with the quality of cooling water, temperature, water velocity, and the magnitude of the biofouling problem. Consequently, the minimum amount of chlorine needed for adequate biofouling control is site-specific and can vary from season to season. Because waters high in organic matter or inorganic reducing agents will rapidly diminish the levels of the biocidally active form of chlorine through oxidation reactions (chlorine-demand reactions), this necessitates the use of higher initial concentrations.
The chlorination procedures necessary for the control of macrobiological fouling depend on the type of organisms which must be eliminated. To control soft-bodied organisms such as hydroids, sponges, and bryozoans, chlorine doses sufficiently high enough to generate residuals of 1 to 3 mg/l, and applied three times per day for 1 to 2 hours have been used. To control hard-shelled organisms such as mussels, low-level continuous chlorination (producing residuals of 0.02 to 1.0 mg/l), particularly during seasons when the animals reproduce, has been found to be effective.
The conventional chlorination practice for macrobiofouling control relies on flushing large volumes of chlorinated water through the circulating water system and on maintaining a free chlorine residual at the condenser outlet, as shown in FIG. 1. Part of the injected chlorine is consumed as the biological chlorine demand in reactions with the macrobiogrowth at the solid surface, some is consumed in the chlorine demand reactions of the water in the boundary layer, and some is convected and diffused in the directions parallel to and normal to the solid surface. Chlorine, convected and diffused normal to the wall, is diluted with the main water flow and consumed in chlorine demand reactions of the main cooling water flow. Chlorine, convected and diffused parallel to the wall, is gradually consumed in the biological chlorine demand reactions with the macrobiogrowth at the solid wall, and in the chlorine demand reactions of the water in the boundary layer. Therefore, at some distance from the injection point, the chlorine residual will reach a value below which the macrobiofouling control is not effective. If the distance between the injection point and the point at which the minimum chlorine residual is maintained increases, the applied chlorine dose must increase to compensate for the larger losses, and vice versa. Only that part of the chlorine dose which reacts with the biogrowth at the wall is "responsible" for antifouling control. Chlorine consumed in the demand reactions represents the loss.
An improved chlorination practice for macrobiofouling control is to maintain a free residual at the condenser inlet box, as shown in FIG. 2.
The applied chlorine dose can further be reduced in cases where macrobiofouling of the intake conduit does not occur due to high water velocities. In such cases, for macrobiofouling control it would be sufficient to maintain a free chlorine residual at the conduit inlet, as shown in FIG. 3.
Spatial variations of residual chlorine concentration in the power plant cooling systems of FIGS. 1-3 are shown in FIG. 4. Since the chlorine demand of the water in the condenser is not treated, the applied chlorine dose and the residual in the discharge are lower for the system of FIG. 2 than for the system of FIG. 1, as evident from FIG. 4. The applied chlorine dose is reduced in the system of FIG. 3 because the chlorine demand of the water in the conduit and in the condenser is not treated. Because of this, the total residual concentration at the compliance point is lower than for the other conventional chlorination practices.
Thus, as is evident from FIGS. 1-3, the conventional chlorination practices are to inject chlorine into the flow near the wall and to maintain the chlorine residual farther along in the main flow, either at the condenser exit or inlet, or at the conduit inlet. The chlorination practices described above rely on treating the entire water volume with chlorine. The total amount of chlorine used in the conventional chlorination practices is high because a large portion of the applied chlorine (e.g., oxidation, decomposition, etc.) dose is consumed in the natural chlorine demand reactions within the entire volume of water, and is converted into biologically much less active and effective forms than the free chlorine.